The invention relates to a method of primary forming a material in which a metal is re-shaped in a primary forming operation.
Primary forming methods of this type comprise, inter alia, casting methods such as pressure-diecasting, gravity-diecasting and sand-casting methods, and also forging and drop-forging methods, in which a metal is re-shaped.
In such primary forming methods, the temperature development of the metal is important for better workability and desired microstructural formation. In particular in the case of casting methods in which a liquid metal or (in the case of thixo casting or solid state melting) pasty or softened solid metal solidifies in the primary mold, it is advantageous to control the temperature variation of the metal. For controlling the temperature variation, it is known, for example from DE 195 45 177 A1, to supply heat to the workpiece or metal inductively by an electromagnetic field which is variable over time. In this way, a defined amount of heat can be introduced into the workpiece, it being possible for the locational and temporal distribution of the inductive field to be varied. The changing of the electromagnetic field can in this case be made dependent on the re-shaping state of the workpiece.
Such inductive heating of a workpiece is already enough to avoid local solidification and undesired premature cooling of the workpiece. However, the inductive coupling-in of the electromagnetic waves can be problematical when relatively large molds are used, for example of a relatively large pressure diecasting installation with metallic mold halves, since the electromagnetic field that changes or oscillates over time has the effect of also inducing eddy currents in the mold halves.
The invention is based on the object of providing improvements over the prior art and, in particular, of providing novel primary forming methods with which heat can be supplied relatively easily to a metal in a desired way, it advantageously being possible for the supply of heat to be varied in terms of space and time.
This object is achieved according to the invention by a conductively generated voltage being applied to parts of the primary forming device during a filling operation before the primary forming operation and/or during the primary forming operation and/or during a subsequent treatment in the primary mold after the primary forming operation, in such a way that a closed circuit is formed and thermal energy is supplied to the metal conductively, with the closed circuit running through the metal.
According to the invention, the metal is consequently heated conductively, i.e. an electric current is passed through the metal. For this purpose, according to the invention, a voltage is applied to parts suitable for this of the molds between which the metal is located or the primary forming device used.
Conductive heating of this type can be achieved with relatively simple apparatus in comparison with inductive heating, by connecting a current source suitable for this to the desired parts of the device. A welding current source which can deliver the desired high current intensities of, for example, greater than/equal to 100 A at the desired voltage of, for example, less than/equal to 100 V may be used for example as the current source.
According to the invention, the voltage can be input in particular at components which have a large contact surface with the metal, such as for example in the case of a casting device the casting plunger, the casting chamber or mold halves, so that a relatively uniform supply of heat to the metal can be achieved. Since, with increasing temperature, the electrical resistance of the metal likewise increases, a natural inverse feedback occurs in the metal, so that, when there is stronger heating of some regions of the metal, the resistance increases and the current flows more through other, cooler regions of the metal.
The voltage can be applied in principle in all regions of the primary forming device. If metallic components are used, in the case of a pressure diecasting device, for example, the casting plunger, the casting chamber, the anvil, the mold halves or a slide, the voltage can be connected directly to these components. Furthermore, it is possible to provide in the components concerned electrodes for the connection of the voltage source, which according to the invention represent parts of the primary forming device. Use of electrodes may, for example, be meaningful in mold halves which have a ceramic insulation with respect to the mold cavity.
The current may be supplied differently in individual phases of the method. In the case of a pressure diecasting method, a voltage can be advantageously applied between the casting plunger and the anvil during the filling phase, in which the molten or pasty metal is filled into the casting chamber, so that a current is generated through the metal over the length of the casting chamber.
In this way it is possible in particular for the temperature of the metal to be kept constant; furthermore, it is possible to increase the temperature of the metal in the casting chamber, so that an unnecessarily great amount of energy is not used up in a melting phase before filling. Later heating is meaningful in particular in the case of thixo-casting methods or SMS (Solid State Melting) methods, in which a slug of a merely softened or slightly pasty metal is introduced, since unnecessary heating of the slug or keeping-warm of preheated slugs before filling can consequently be at least partially prevented.
During mold filling, a voltage can be applied in particular between the mold halves, so that the metal located in the mold cavity is heated directly between the mold halves. If the mold halves are covered on their inner side to a relatively great extent with a ceramic insulator, electrodes can be additionally used for this purpose. Metallic cooling elements fitted in the mold halves may also be used, for example, as electrodes, so that it is unnecessary for further electrodes to be fitted in existing devices. Furthermore, a current can also be generated between the casting plunger or casting chamber and one or both mold halves, so that the metal still in the casting chamber and in the gate is also heated. If slides are used for producing desired pressure diecasting molds, these can also be used as a connecting point for voltage with respect to another slide, one or both mold halves or, in particular, the casting plunger and/or casting chamber.
In the solidifying phase after mold filling, it may be advantageous in particular to keep the butt of material between the casting plunger and the anvil liquid, in order that a pressure continues to be exerted on the metal located in the mold cavity, in order to ensure good compaction or dense feeding of the metal in the mold cavity. Furthermore, it is possible, for example by a voltage between the casting plunger or casting chamber and the mold halves, or an electrode fitted in them or a slide fitted in them, to keep clear the gate which joins the space inside the casting chamber to the mold cavity, in order that the pressure exerted on the butt of material acts for a longer time on the metal located in the mold cavity.
Furthermore, by suitable current conduction in the metal, it is possible to achieve directional solidification in the mold cavity, in that cooling initially occurs in the regions of lower current intensity and then spreads from these regions into the mold cavity. Since, where there is cooling, the metal is detached from the walls of the mold halves, a sudden change in resistance occurs, which further intensifies this effect. Consequently, a detachment of the metal in the solidified regions from the mold halves and directional solidification can be gradually achieved.
After the solidifying process, a further thermal treatment, in which thermal energy is conductively supplied to the material in desired regions in a desired way, can be carried out without further aids to improve the microstructure. For this purpose, electrodes, for example cores, preferably embedded in the cast piece, can be used, whereby it is ensured that current is also supplied in regions of the metal which have become detached from the mold halves.
The method according to the invention can be carried out manually. Furthermore, the voltage or current variation can be varied in dependence on calculated or measured function curves. For this, a desired starting-up or switching-off characteristic may be implemented, for example, when raising and lowering the current intensities. According to the invention, automatic control with temperature measurement of the metal is also advantageously possible, so that the voltage or current intensity can be changed in dependence on a temperature or the temperature values measured at a number of points of the metal. This allows the temperature to be kept constant, in particular, or a lowering of the temperature of the metal to be delayed. Furthermore, another measured variable may also be measured for an automatic control procedure. This may, in particular, involve measuring an internal mold pressure between the mold halves. This measurement may take place, for example, by means of a membrane which is provided flush in the mold surface or inner face of the mold halves and is connected to a piezoelectric crystal fitted outside the mold cavity. As an alternative to this, a piezoelectric crystal may be provided under an ejector pin, so that a measurement of the internal mold pressure is possible by means of the displaceable ejector pin.
Furthermore, in addition or as an alternative to this, a sound emission, in particular an ultrasound emission of the metal, for example in the range of 100 KHZ, may be measured. A sound emission analysis of this type can be used to establish the undesired cavitation taking place during the solidifying process. An example of such sound emission analysis is shown in DE 39 40 560 C2. According to the invention, the automatic control takes place here in such a way that, when a measuring signal or excessively high measuring signal is obtained, the heating power is increased, so that the measured signal is kept to a minimal level.